This invention relates to an organic electroluminescent (OEL) element, an OEL panel and an OEL apparatus using the same. The OEL element and the OEL panel are used for a display device, a light source of a display device, a backlight, a light-emitting device used in an optical communication apparatus and the like.
An electroluminescent element is a light-emitting device utilizing electroluminescence of solid fluorescent material. An inorganic electroluminescent element using inorganic material as a light-emitting device has been commercialized and has been used for a backlight of a liquid crystal display and so on. On the other hand, an electroluminescent element using organic material has been studied for years, but has not yet been in the actual use, because its luminous efficiency is very low.
A multilayer type organic electroluminescent (OEL) element was reported by C. W. Tang et al. in Eastman Kodak Company in 1987, where the OEL element was formed of two layers (a hole transporting layer and an emitting layer). According to the report, the OEL element showed high light brightness (at least 1000 cd/m2) even if its driving voltage was low (as low as 10V). [cf. C. W. Tang and S. A. Van slyke: Apply. Phys. Lett., 51, 913 (1987)]. Since then, OEL elements have drawn additional attention, and research and development involving OEL elements has increased. As a result of this activity, OEL elements have been commercialized.
A structure of a conventional OEL element is described hereinafter with reference to FIG. 8. As shown in FIG. 8, an OEL element includes anode 2, organic hole-transporting layer 4, organic light emitting layer 5 and cathode 6, where anode 2 is formed on transparent substrate 1, e.g., glass. Generally, anode 2 is a transparent conductive layer, e.g., indium tin oxide (ITO), formed by a sputtering method or a resistor-heating vacuum deposition and the like. Hole transporting layer 4 is a deposited organic thin layer, e.g., N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-bis(3-methylphenyl)-1,1xe2x80x2-diphenyl-4,4xe2x80x2-diamine(TPD). Organic light emitting layer 5 is a deposited organic thin layer, e.g., 8-Hydroxyquinoline Aluminum (Alq 3). Cathode 6 is a metal layer whose thickness ranges from 100 nm to 300 nm.
When a DC voltage is applied between anode 2 and cathode 6 of the OEL element, holes are injected from anode 2 to emitting layer 5 via hole-transporting layer 4, and electrons are injected from cathode 6 to emitting layer 5. Holes and electrons are recombined in emitting layer 5, and excitons are produced. Light emission occurs when excitons transmit from an excited state to a ground state.
When the OEL element is used for a display device, it can be applied for a dot matrix panel whose pixels are arrayed in a matrix, where the pixels are formed of an anode, organic material and a cathode.
Two driving methods of the dot matrix panel have been proposed. One is an active matrix method (AM method) which is disclosed in Japanese Patent Non-Examined Publication No. H09-139286. In accordance with the AM method, every pixel has a thin film transistor (TFT) and is controlled individually. Another method is a passive matrix method (PM method) which uses no TFTs. In the PM method, a panel is driven line by line sequentially by the PM method.
The OEL element can be used for both methods because the response of the OEL is very fast. The PM methodxe2x80x94differing from the AM methodxe2x80x94needs not to have a TFT in every pixel. The number of manufacturing processes of the PM method are fewer than that of the AM method, and a production cost of the PM method can be lower than that of the AM method. From a viewpoint of power consumption, however, the AM method is advantageous, particularly, the more the number of pixels increases, the more the AM method become advantageous.
The reason of increasing power consumption by the PM method is described hereinafter with reference to FIG. 9. As shown in FIG. 9, striped anode 2 crosses cathode 6 at right angles. Organic thin film 3 is sandwiched between two electrodes (anode 2 and cathode 6), so that the OEL element is formed. The OEL element emits light at an intersection, i.e., single pixel 500, of the electrodes. FIG. 9 shows a schematic view of a conventional display panel by the PM method. A plurality of pixels 500 are arrayed in matrix on the display. Respective scanning lines (cathode lines in FIG. 9) are scanned sequentially, and at that time, a data line corresponding to a pixel desired to emit needs to be turned on. As a result, an arbitrary pixel for display emits light.
In the PM method, a time span, in which one line of the OEL emits, is described as follows:
flame time/the number of scanning lines and a duty ratio is described as follows:
1/the number of scanning lines
When the number of scanning lines is increased, instantaneous brightness, at which one line of the OEL emits during the time span, needs to be increased proportionately.
In an OEL element, a driving current is generally proportional to brightness, when the brightness is low. When the number of scanning lines is increased, more the current is required, because high instantaneous brightness is needed. When emission brightness is high, the proportionality mentioned above does not hold, so that much more current is needed. On the other hand, in the AM method, high instantaneous brightness is not required because every pixel has a TFT. The more the number of pixels increases, the greater a difference of power consumption between the AM method and the PM method becomes.
When the number of pixels increases, crosstalk tends to appear when utilizing the PM method. Crosstalk is a phenomenon in which not only a selected line but also an unselected line around the selected line emits light. Another electric power is needed to reduce the crosstalk besides the electric power for emitting. A reverse-bias method, one of measures against the crosstalk, is disclosed in Japanese Patent Non-Examined Publication No. H04-308687. In the reverse-bias method, the voltage is applied to an unselected line in a reverse direction of the selected line (emission line). Electric power consumed by the reverse-bias method causes the increase of power consumption by the PM method. The more the number of pixels increases, the greater a difference of power consumption between the AM method and the PM method becomes.
One of the measures for reducing power consumption is to lower the instantaneous brightness. For example, the method, by which a display area is divided into more than two areas and the divided areas are driven respectively, is disclosed in Japanese Patent Application Non-Examined Publication No. 2000-29432. By the method mentioned above, the number of electric wirings increases responsive to the increase of the number of divided sections. As a result, the space required for electric wirings becomes large and an emission area is reduced.
The present invention addresses the problems discussed above, and aims to provide an inexpensive organic electroluminescent (OEL) element for a dot matrix panel consuming lower power. Accordingly, this invention provides an OEL panel and an OEL apparatus using the OEL element.
An exemplary OEL element of this invention includes a scanning electrode, an organic thin film and a signal electrode.
The signal electrodes are formed of N-layer laminated electrodes, where respective layers are insulated from each other. Layer M is formed on layer (Mxe2x88x921) via the insulator, where M is an integer not more than an integer N (N greater than M) and greater than 1, and where the area of layer M is smaller than that of layer (Mxe2x88x921). The organic thin film and the scanning electrode are formed on layer N and on layer (Mxe2x88x921) not covered with layer M. As a result, one substrate is divided into N sections and the divided sections are scanned respectively. Thus, the duty ratio for driving the OEL element becomes larger and power consumption becomes smaller.
The OEL element of the present invention can be easily coupled to an external circuit and the reliability of the OEL element is improved because all of lead-wires from the signal electrodes are routed in the same direction on the substrate.
An OEL apparatus of the present invention includes at least one driving IC and the driving IC scans each divided section independently, where one display area in the OEL element is divided into sections.
Several OEL elements can be arrayed on one substrate, so that a display area is divided into a greater number of sections, and a duty ratio for driving the OEL element becomes larger and therefore less power consumption can be expected.